1. Field of the Invention
The present invention relates to a liquid crystal display having a reflector disposed outside a liquid crystal cell. In particular, the present invention relates to a liquid crystal display that achieves a wide viewing angle, a high bright display, and a high contrast without having to provide a retarder between the liquid crystal panel and the reflector outside the liquid crystal panel.
2. Description of the Related Art
Generally, liquid crystal displays can be classified into transflective liquid crystal displays provided with backlights, transmissive liquid crystal displays, and reflective liquid crystal displays depending on the display mode used. Reflective liquid crystal displays do not use backlights and use only external light, such as sunlight and illuminating light. Reflective liquid crystal displays are frequently used in thin, lightweight, portable information terminals that require low power consumption, for example. Reflective liquid crystal displays are provided with reflectors for reflecting the light entering from the display surface. Plates having mirror-finished surfaces or plates having irregular surfaces have been used as the reflectors.
FIG. 19 is a cross-sectional view showing the overall structure of a known reflective liquid crystal display 50 provided with a reflector that has an irregular surface. The reflective liquid crystal display 50 includes a liquid crystal panel 50a comprising: a pair of glass substrates 51 and 52 that face each other; a transparent electrode layer 53 provided on the face of the glass substrate 51 that opposes the glass substrate 52; a transparent electrode layer 54 provided on the face of the glass substrate 52 that opposes the glass substrate 51; an alignment film 55 for aligning liquid crystal, the alignment film 55 being disposed on the transparent electrode layer 53; an alignment film 56 for aligning liquid crystal, the alignment film 56 being disposed on the transparent electrode layer 54; and a liquid crystal layer 57 disposed between the alignment films 55 and 56. The liquid crystal layer 57 is sealed in the gap between the glass substrates 51 and 52 with a sealant 65.
A first retarder 66 is disposed on the outer face of the glass substrate 51 of the liquid crystal panel 50a. A second retarder 67 is disposed on the first retarder 66, and a first polarizer 68 is disposed on the second retarder 67. A second polarizer 69 is disposed on the outer face of the glass substrate 52 of the liquid crystal panel 50a, and a reflector 70 is attached to the outer face of the second polarizer 69 with a transparent bonding layer 70a therebetween.
As shown in FIG. 20, the reflector 70 is, for example, made by sand-blasting the surface of a resin film 71 to form an irregular surface and then forming a reflecting layer 72 on this irregular surface by deposition using aluminum or the like. The reflector 70 is positioned so that the surface provided with the reflecting layer 72 opposes the second polarizer 69.
In the reflective liquid crystal display 50 having the above structure, light incident on the first polarizer 68 is linearly polarized by the first polarizer 68. The linearly polarized light then becomes elliptically polarized as the light is transmitted through the second retarder 67, the first retarder 66, and the liquid crystal layer 57. The elliptically polarized light then becomes linearly polarized as the light is transmitted through the second polarizer 69. The linearly polarized light is then reflected at the reflector 70, transmitted through the second polarizer 69, the liquid crystal layer 57, the first retarder 66, and the second retarder 67, and is emitted from the first polarizer 68.
Generally, liquid crystal displays desirably have good display characteristics such as resolution, contrast, display brightness, and visibility such as a wide viewing angle.
However, in the conventional reflective liquid crystal display 50, because the irregular surface of the reflector 70 subjected to sand-blasting has a poor reflection efficiency, the overall reflectance of the reflective liquid crystal display 50 is low. In other words, the conventional reflective liquid crystal display 50 fails to meet the needs for a reflector that reflects incident light over a greater range of reflection angles. The viewing angle of the reflective liquid crystal display 50 including this reflector 70 is relatively small, i.e., approximately 25 to 35 degrees; furthermore, the brightness of the display is not sufficient.
In contrast to the above-described reflector 70 having an irregular surface, a mirror reflector shows a sharp, high reflectance at a particular angle of reflection (specular angle of reflection) with respect to the angle of incidence. However, the range of reflection angles exhibiting a high reflectance is excessively narrow. As a result, a conventional reflective liquid crystal display having a mirror reflector has a problem of narrow viewing angle.
An attempt has been made to improve the brightness of the white display during application of selection voltages by reducing the number of the polarizer used in the liquid crystal display. That is, the second polarizer 69 between the glass substrate 52 and the reflector 70 is removed, and only the first polarizer 68 disposed on the second retarder 67 is used. However, in such a reflective liquid crystal display in which one of the polarizers is removed, the reflection efficiency of the reflector 70 is still low. Thus, both the white display and the black display become bright, thereby degrading the contrast.
The present inventors have been studying the type of reflector shown in FIG. 21. A reflector 51 shown in FIG. 21 is, for example, made by sequentially forming many concavities 54 in a resin substrate 53 composed of photosensitive resin or the like disposed on a substrate 52 composed of glass or the like, each of the concavities 54 having a curved inner surface that represents part of a spherical surface, and forming a reflecting film 55 by vapor deposition or printing using, for example, aluminum or silver, on the resin substrate 53 provided with the concavities 54.
The concavities 54 are formed at random at a depth ranging from 0.1 to 3 μm. The pitch between adjacent concavities 34 is also randomly set within the range of 5 to 50 μm. The inner surface of each of the concavities 54 is curved and represents part of a spherical surface. The angle of the slope of the curved inner surface is set within the range of −18 to +18.
Note that in this specification, the term “depth of the concavity” is defined as the distance between the surface of the substrate of the reflector and the bottom of the concavity. The term “pitch between adjacent concavities” is defined as the distance between the centers of the circles of the adjacent concavities in a plan view of the concavities 54. The term “tilt angle” is defined as an angle formed by a tangential line and the substrate surface at any point of the inner surface of the concavity 54 in a particular vertical cross-section.
The reflector 51 shown in FIG. 21 has the reflection characteristic shown in FIG. 16. In FIG. 16, the reflection characteristic of the reflector 51 is shown as that of the Comparative Example described below. FIG. 16 is a graph showing a reflection characteristic curve at an incident angle of 30 degrees. In the graph, the vertical axis shows the reflectance (reflection intensity), and the horizontal axis shows the angle of reflection.
As shown in FIG. 22, the angle of incidence is the angle ω0 between a normal line H perpendicular to the reflector 51 (the substrate surface) and incident light J. The angle of reflectance is an angle ω between the normal line H and the reflected light K in a plane that includes the normal line H and the incident light J. The angle of specular reflectance is the angle at which the incident angle ω0 is equal to the reflectance angle ω.
As shown in FIG. 16, the reflector 51 has a sufficient reflectance around a reflection angle of 30 degrees, i.e., the angle of specular reflection, and more particularly in the range of 15°≦ω≦45°.
The conventional reflector 51 described above exhibits a relatively good reflectance over a relatively wide range of angles because of the concavities 54. However, as shown in FIG. 16, the reflectance peaks at reflection angles of 15 and 45 degrees and is low around a reflection angle of 30 degrees, i.e., the angle of specular reflection. Accordingly, although the reflector 51 achieves a relatively good reflectance over a particular range, the brightness is slightly degraded in the direction of the specular reflection.
When a display device is mounted in a particular device such as a notebook computer, a desk calculator, or a wristwatch, the angle between the direction of a light source and the display device, i.e., the angle of incidence, and the angle between the line of sight of a user who receives the reflected light and the display device, i.e., the angle of reflection or the receiving angle, are usually within specific ranges. Users desire an increase in the reflection intensity in a particular direction as well as a bright display over a wide range of angles.
Furthermore, when the above reflector 51 that achieves bright display over a wide range of angles is used in a liquid crystal device provided with a backlight, the surface of the reflector 51 excessively scatters the light from the backlight, thereby reducing the amount of light emitted in the direction most advantageous for the users.